1. Field of the Invention
The present invention relates to an Mnxe2x80x94Zn ferrite for power cores such as those for power transformers operating at a high frequency of the order of 10 to 500 kHz in particular, and a core for power supplies made up of this ferrite.
2. Prior Art
Manganese-zinc ferrites find widespread use in the form of core materials for coils and transformers in a variety of communications systems, consumer-oriented electronic systems, etc. In recent years, however, power supplies of higher frequencies have been increasingly used and so core materials have been required to have performance fit for this purpose. Needed for switching power supplies in particular are transformers that are used with a few tens of watts in a high frequency region of 10 to 500 kHz. Besides, cores for various transformers for the purposes of motor driving, signal amplification, oscillation, etc. are in need. So far, manganese-zinc type low-loss ferrites have been used for transformer cores. However, improvements in the power losses, called core losses, of these ferrites are now demanded because the core losses are increased in a high frequency region of the order of 10 to 500 kHz. To this end various proposals have been made.
Among these proposals there is one where oxides of Si, and Ca are used with the addition thereto of oxides of tetra-valent metals such as Sn, Ti and Zr or oxides of penta-valent metals such as V, Nb and Ta. Examples of the sole or combined addition of oxides of tetra- or penta-valent metals are set forth in JP-A""s 46-2880, 48-72696, 60-262404, 61-108109, 61-252609, 61-252611 and 63-222018 as well as JP-A""s 01-129403, 02-54902, 03-141611, 03-163804, 03-223119, 03-248403, 03-248404, 03-248405, 03-254103, 04-55362, 04-150007, 05-198416 and 05-267040.
With these, however, it is impossible to decrease power losses at high frequencies, e.g., at 100 kHz and 100xc2x0 C. Accordingly, when these conventional ferrites are used, it is difficult to reduce the size of transformers.
JP-A 06-5411 discloses an Mnxe2x80x94Zn ferrite comprising CaO and SiO2 and further containing at least one of Nb2O5 and V2O5. The cooling rate in the sintering process is controlled to 60xc2x0 C./hour to 550xc2x0 C./hour inclusive in the temperature range from the temperature at which the atmosphere is changed over to 100% nitrogen to 800xc2x0 C., whereby the minimum temperature for power losses can be freely regulated to 60 to 120xc2x0 C. and power losses can be reduced.
However, this publication has no consideration of the temperature at which the atmosphere is changed over to 100% nitrogen, and so refers to only 1,150xc2x0 C. in the example. The resultant power losses are at most 310 kW/m3 at 100xc2x0 C.; they are still less than satisfactory.
An object of the present invention is to provide a process for the production of a ferrite with reduced magnetic losses and power losses, a ferrite obtained by this production process, and a core for power supplies using this ferrite.
Such an object is achieved by the following embodiments (1) to (11) of the present invention.
(1) An Mnxe2x80x94Zn ferrite production process comprising a maximum temperature holding step for firing and a cooling step in a nitrogen atmosphere, wherein:
a nitrogen atmosphere changeover temperature T in said cooling step is below 1,1500xc2x0 C. and equal to or higher than 1,000xc2x0 C., and a cooling rate V1 conforms to a condition defined by the following formula (1):
Txe2x89xa6(V1+1,450)/1.5xe2x80x83xe2x80x83(1)
where T is the nitrogen atmosphere changeover temperature in xc2x0 C. and V1 is the cooling rate in xc2x0 C./hour from T down to 900xc2x0 C.
(2) An Mnxe2x80x94Zn ferrite production process comprising a maximum temperature holding step for firing and a cooling step in a nitrogen atmosphere, wherein:
a nitrogen atmosphere changeover temperature T in said cooling step is below 1,0000xc2x0 C. and equal to or higher than 900xc2x0 C., and a cooling rate V1 conforms to a condition defined by the following formula (2):
Txe2x89xa6(V1+450)/0.5xe2x80x83xe2x80x83(2)
where T is the nitrogen atmosphere changeover temperature in xc2x0 C. and V1 is the cooling rate in xc2x0 C./hour from T down to 900xc2x0 C.
(3) The Mnxe2x80x94Zn ferrite production process according to (1) or (2) above, wherein said cooling rate V1 from said nitrogen atmosphere changeover temperature T down to 900xc2x0 C. is 800xc2x0 C./hour or less.
(4) The Mnxe2x80x94Zn ferrite production process according to any one of (1) to (3) above, wherein a cooling rate V2 from 900xc2x0 C. down to 600xc2x0 C. is 200 to 800xc2x0 C./hour.
(5) The Mnxe2x80x94Zn ferrite production process according to any one of (1) to (4) above, wherein a maximum temperature in said maximum temperature holding step is 1,250 to 1,350xc2x0 C. while a maximum temperature holding time is 2 to 7 hours.
(6) The Mnxe2x80x94Zn ferrite production process according to any one of (1) to (5) above, wherein:
said Mnxe2x80x94Zn ferrite comprises as main components 52 to 55 mol % of iron oxide as calculated on an Fe2O3 basis and 7 to 12 mol % of zinc oxide as calculated on a ZnO basis with the balance being manganese oxide, and contains as subordinate components 60 to 140 ppm of silicon oxide as calculated on an SiO2 basis, 350 to 700 ppm of calcium oxide as calculated on a CaO basis and 100 to 350 ppm of niobium oxide as calculated on an Nb2O5 basis.
(7) The Mnxe2x80x94Zn ferrite production process according to (6) above, wherein zirconium oxide is contained as an additional subordinate component in an amount of 50 to 350 ppm as calculated on a ZrO2 basis with respect to said main components.
(8) The Mnxe2x80x94Zn ferrite production process according to (6) or (7) above, wherein nickel oxide is contained as a further subordinate component in an amount of 0 to 4,500 ppm, exclusive of 0, as calculated on an NiO basis with respect to said main components.
(9) The Mnxe2x80x94Zn ferrite production process according to any one of (6) to (8) above, wherein a phosphorus content is 30 ppm or less as calculated on a P basis with respect to said main components.
(10) The Mnxe2x80x94Zn ferrite obtained by any one of the processes recited in (1) to (9) above, and according to any one of (1) to (5) above, wherein a power loss of 280 kW/m3 or less is achieved at an applied AC magnetic field of 100 kHz and 200 mT, as measured at a temperature of 100xc2x0 C.
(11) A ferrite core for power supplies, made up of the Mnxe2x80x94Zn ferrite according to (10) above.